Generally, drugs administered to the living body reach target sites and cause therapeutic effects by exerting pharmacological effects at the localized target sites. However, there will not be a cure if drugs reach tissues other than the target sites (that is, normal tissues). Consequently, how to guide drugs to the target sites efficiently is important in terms of therapeutic strategy. Such a technology for guiding drugs to the target sites is called drug delivery and research and development thereof have been actively carried out in recent years. These drug delivery methods have at least two merits. One is that a sufficiently high drug concentration is obtained in affected tissues. This is advantageous because pharmacological effects are achieved only when the drug concentration in the target site is higher than a certain value, and therapeutic effects can not be expected when the concentration is low. Second is that the drug delivery methods guide drugs to affected tissues only and do not guide drugs to normal tissues unnecessarily. Side effects can thereby be suppressed.
Such drug delivery methods exert their effects most in cancer treatments using anticancer agents. Since most anticancer agents suppress cell growth of cancer cells which are actively dividing, they also suppress cell growth in normal tissues where cells are actively dividing such as, for example, bone marrow, hair-roots, or gastrointestinal mucosa. On this account, side effects such as anemia, hair loss, and vomiting appear in cancer patients who have received administration of anticancer agents. Dosage has to be restricted since these side effects would be heavy burdens on patients and thus, there is a problem in that pharmacological effects of anticancer agents cannot be obtained sufficiently. Furthermore, there is a concern of patients dying due to the side effects in worst cases. Accordingly, it is hoped that cancer treatments can be carried out efficiently while suppressing the side effects by guiding the anticancer agents until they reach cancer cells with drug delivery methods and allowing the agents to exert their pharmacological effects on cancer cells, specifically.
Apart from anticancer agents, for example, application of the drug delivery methods to agents for treating male erectile dysfunction is considered. There are examples of significant systemic hypotension resulting in deaths caused by the use of agents for treating male erectile dysfunction when combined with nitro preparations and thus, it is a problem particularly for males of middle and old age with heart disease. This is because the agents for treating erectile dysfunction do not necessarily concentrate at the target site, act on systemic blood vessels, and thereby increase vasodilation effects of nitro preparations. Accordingly, it is considered that the side effects resulting from the combined use with nitro preparations can be suppressed by guiding the agents for treating male erectile dysfunction until they reach the target site with drug delivery methods and allowing the agents to exert their pharmacological effects on the target site specifically.
As a specific method of drug delivery methods, for example, guidance to the target site using supports (carriers) is being studied and this method is to load drugs onto supports that tend to concentrate in the target site and thereby make the supports transport the drugs to the target site. As supports, use of various types of antibodies, microspheres, or magnetic bodies has been discussed. Among them, magnetic bodies are those that are regarded as particularly hopeful and a method to attach the supports, which are magnetic bodies, to the drugs and make them accumulate in the target site by means of a magnetic field has been examined (for example, refer to the following Patent Document 1). Since this guiding method is easy and simple and makes treatment which targets the target site possible, it is considered to be an effective method especially for anticancer agents with high cytotoxicity.    Patent Document 1: Japanese Laid-Open Patent Application No. 2001-10978